The wireless industry is en-route to another revolution with the advancement of millimeter-wave technologies and the explosion of every day electronic devices connected to the Internet in the new age of the Internet-of-Things (IoT). Since the publication of “Millimeter Wave Mobile Communications for 5G Cellular: It will Work” in May 2013 in IEEE ACCESS, there has been worldwide interest in using millimeter wave (mmW) wireless spectrum for future mobile and portable communications. An expanding use of wirelessly connected devices has created a spectrum and capacity crunch that has led industry and academia to search for near-term solutions and technologies. Millimeter wave (mmW) frequencies between 30 and 300 GHz (and actually below 10 GHz, as well, including bands such as 28, 37-39, 57-63, 64-71, 71-76, and 81-86 GHz, have been attracting growing attention as a possible candidate for next-generation microcellular networks due to the availability of unused spectrum. These higher frequencies provide much wider bandwidths than current cellular and their feasibility has more or less been proven as a viable solution for mobile, backhaul, and indoor wireless communications through recent research. Furthermore, the Federal Communications Commission (FCC) in the United States has put forth a petition to investigate and consider the use of frequencies above 24 GHz for future wireless networks (FCC Proceeding 14-177). Exemplary embodiments of such mmW systems can be based on orthogonal frequency division multiplexing (OFDM) technology that is known to persons skilled in the art. More background information concerning mmW technology can be found in T. Rapaport, et al., MILLIMETER WAVE WIRELESS COMMUNICATIONS (Prentice-Hall 2014).
However, currently available mmW devices principally use highly directional horn antennas to enable short-range, line-of-sight links, within a controlled and immobile environment, such as a point to point link between buildings or in or between a data center. Since such an ideal environment and line of sight conditions are very difficult—if not impossible—to replicate in a practical system implementation where mobile users are involved, there is a need for building mmW systems “in the wild,” e.g., where line-of-sight is not always available, SNRs are lower, and where mobility is the norm, so the use of static directional antennas is infeasible.
Accordingly, mmW systems and devices are likely to utilize a variety of multi-antenna technology (e.g., antenna arrays) at the transmitter, the receiver, or both. Currently, arrays used in base stations and mobile stations for transmission and/or reception of cellular-band (e.g., 1-2 GHz) signals are limited to a few elements, e.g., two to six. However, the small wavelengths of the mmW bands, combined with advances in radio-frequency (RF) electronics, have facilitated mmW arrays containing a large number of antenna elements to be fabricated at costs suitable for large-volume consumer devices. Certain exemplary mmW array designs envision 16 to 64 antenna elements in both a fixed device (e.g., access point or base station) and in a mobile or portable device (e.g., smartphone or tablet). Such an exemplary design is described in S. Shu, et al., “MIMO for Millimeter-wave Wireless Communications: Beamforming, Spatial Multiplexing, or Both?”, IEEE COMM'NS MAG. 110-21, December 2014.
Multi-antenna technology can be used to improve various aspects of a communication system, including system capacity (e.g., more users per unit bandwidth per unit area), coverage (e.g., larger area for given bandwidth and number of users), and increased per-user data rate (e.g., for a given bandwidth and area). Directional communications using multiple antenna can also ensure better wireless links as a mobile or fixed devices experience a time-varying channel.
For example, multiple antennas at the transmitter and/or the receiver can be used to shape or “form” the overall antenna beam (e.g., transmit and/or receive beam, respectively) in a particular way, with the general goal being to improve the received signal-to-interference-plus-noise ratio (SINR) and, ultimately, system capacity and/or coverage. This can be accomplished, for example, by maximizing the overall antenna gain in the direction of the target receiver or transmitter or by suppressing specific dominant interfering signals, or by using a few different beams that contain significant energy. In general, beamforming can increase the signal strength at the receiver in proportion to the number of transmit antennas, and may also null out or minimize interference. Beamforming can be based either on high or low fading correlation between the antennas. High mutual antenna correlation can typically result from a small distance between antennas in an array. In such exemplary conditions, beamforming can boost the received signal strength but does not provide any protection against radio-channel fading, because such an arrangement does not provide diversity. On the other hand, low mutual antenna correlation typically can result from either a sufficiently large inter-antenna spacing or different polarization directions in the array. If some knowledge of the downlink channels of the different transmit antennas (e.g., the relative channel phases) is available at the transmitter, multiple transmit antennas with low mutual correlation can both provide diversity, and also shape the antenna beam in the direction of the target receiver and/or transmitter.
By way of a further example, multiple antennas at both the transmitter and the receiver can improve the SINR and/or achieve additional diversity/protection against fading compared to only multiple receive antennas or multiple transmit antennas. This can be useful in relatively poor channels that are limited, for example, by interference and/or noise (e.g., high user load or near cell edge). In relatively good channel conditions, however, the capacity of the channel becomes saturated such that further improving the SINR provides limited increases in capacity. In such exemplary cases, using multiple antennas at both the transmitter and receiver can be used to create multiple parallel communication “channels” over the radio interface. This can facilitate a highly efficient utilization of both the available transmit power and the available bandwidth resulting in, e.g., very high data rates within a limited bandwidth without a disproportionate degradation in coverage. For example, under certain exemplary conditions, the channel capacity can increase linearly with the number of antennas and avoid saturation in the data capacity and/or rates. These techniques are commonly referred to as “spatial multiplexing” or multiple-input, multiple-output (MIMO) antenna processing.
One challenge in designing and building such multi-antenna devices and systems can be that they must be thoroughly tested for reliability, functionality, and/or performance during development (e.g., in a design and/or qualification laboratory) and/or manufacturing. For example, testing wireless devices can be challenging due to the unique characteristics and vagaries of the wireless channel. A channel emulator is a device that re-creates a physical channel between the TX and RX devices for different geographical conditions, under various multipath, mobility, weather, and fading scenarios. It can be a staple part of any laboratory or factory where wireless devices are designed and/or tested. Typically, the TX and RX devices under test (DUTs) are connected to the channel emulator using cables that carry the RF signals. The wireless devices can then be tested based on the emulated wireless channel.
To be useful in this manner, however, channel emulators can require accurate models of each physical channel to be applied to the DUTs. Existing models for lower-band channels used in 3G and 4G/LTE systems were created in part by measurements contributed to the Third-Generation Partnership Project (3GPP) and WINNER-II initiative. Such models can be constructed from channel measurements made using channel measurement devices and/or systems known as channel sounders. One objective of a channel sounder is to accurately capture one or more parameters of the channel, including but not limited to a complex channel impulse response (CIR) and a power delay profile (PDP). Such parameters generally represent the time delay behavior of the wireless channel between a transmitter and receiver, together with accurate measures of power decay and multipath characteristics as a function of time delay.
As described in T. Rappaport, WIRELESS COMMUNICATIONS: PRINCIPLES AND PRACTICES (2d ed., 2002), a power delay profile is a practical measure of the time delays based on a finite bandwidth probe transmitted by the channel sounder transmitter where the power of the multipath channel is determined over time delay. Moreover, the square root of a PDP can contain phase information for individual multipath components and, therefore, can be a surrogate for the complex CIR. As used herein, both CIR and PDP can—but do not necessarily—include phase information as well as angular/spatial direction and other channel characteristics known to those skilled in the art.
While existing channel sounders may have been adequate for lower-band channels with smaller bandwidths, they are lacking in the flexibility, measurement bandwidth, and processing capacity necessary to properly measure and construct models for mmW channels where both higher bandwidths and spatial/angular directionality will be critical for proper measurement and modeling of future wireless systems that use highly directional and adaptive antenna arrays in the mobile device or base station/access point. For example, measuring across wide bandwidths at mmW bands can be extremely difficult due to the complexity in synchronizing all frequency sources in a measurement system. Utilizing TX and/or RX MIMO antenna systems also increases complexity, at least in terms of the number of TX-RX paths that must be measured and/or modelled.
Accordingly, there may be a need to address at least some of the inadequacies, issues, and/or concerns with existing channel measurement devices and techniques described herein.